This invention concerns an electron beam cathodoluminescent panel display suitable for the display of television pictures. It is also useful for other image displays such as alphanumeric, computer and computer graphics.
The achievement of a feasible and practical flat panel television display has long been a goal of technologists in many parts of the world. However, to have widespread commercial significance, any such display must be technically and economically competitive with conventional cathode-ray picture tubes.
Such picture tubes are in an advanced state of refinement. In many respects, the attainable picture performance of the picture tube is at such a high level that there is little practical incentive for further technological improvements. Contrast ratios, brightness levels, raster linearity, interlace and color field registration are quite acceptable to television viewers. Resolution, particularly in picture highlight areas, however, generally falls discernibly below theoretical system limits. This impediment is being overcome by new types of high-resolution electron guns coming into use.
Conventional picture tubes do have characteristics that provide incentive to create a commercially viable alternative such as the flat panel display. For example, the picture tube has a very real, practical size limitation. The largest color tubes commonly in current production have a display screen with an approximately 25-inch diagonal measurement, providing about 315 square inches of viewing area. The 25-inch measurement does not represent an absolute physical limit, but there are a number of practical considerations which rule out any major increase. Volume, weight and cost of the picture tube envelope tend to increase very rapidly for even modest increases in picture area. In addition, equivalent brightness and resolution are difficult if not impossible to attain in larger configurations.
In view of these disadvantages, the flat panel display represents a highly attractive alternative. An ideal panel display would provide picture performance equal to or exceeding the present quality levels of the picture tube, and would not be so rigorously size-limited.
A major effort in creating a flat panel display has been directed to the gas-discharge type; however, panels developed to date have not demonstrated adequate efficiency. In view of this fact, the efficiency of the electron beam of the picture tube in activating cathodoluminescence makes the use of such beams highly attractive in a panel display. Also, there is a wealth of readily available picture tube technology that is applicable to a panel display using electron beams; phosphor and high-vacuum technologies are prime examples.
An attempt to utilize the electron beam in a flat panel display is shown by the "Aiken" tube (refer to FIG. 1) wherein a pair of electron guns 10 project beams 12 parallel to two enveloping plates 14 and 16, one of which is transparent. Beams 12 are diverted to fall upon opposite sides of cathodoluminescent surface 18. The beams are diverted by deflection plates 20 and 22, which are used to scan surface 18 in vertical and horizontal directions to produce an image. The concept is covered in a series of U.S. patents by Aiken, including U.S. Pat. No. 3,313,970. The beams are of high energy, and high potentials on the deflection plates are required to divert the beams toward the cathodoluminescent surface, making the tube impractical for consumer product television displays. Color rendition has also been less than ideal. Further, since the envelope is not self-supporting against atmospheric pressure, the concept would seem to be adaptable to only relatively small displays.
Gabor has disclosed a three-beam flat panel color display tube shown in highly simplified schematic form in FIG. 2. Three electron beams 24 are generated by electron guns 26, and turned back one hundred and eighty degrees around barrier 28 into an adjacent beam channel 30, where the beams are diverted again ninety degrees by electrodes 32 to impinge upon and scan cathodoluminescent color phosphor screen 34 through a shadow mask 36. This concept is covered in U.S. Pat. No. 3,171,056, among others. The Gabor tube is a very complex structure which must be made with extreme precision. Beam energies are relatively high, and high deflection potentials are required to scan the beam. It is believed that a complete operative tube has never been made. It is also thought that such a tube, if realizable, would be seriously effected by external influences such as the earth's magnetic field. Like the Aiken tube, it is not a self-supporting structure so its use would also be restricted to relatively small displays.
Charles, in U.S. Pat. No. 3,723,786, discloses a flat cathode-ray tube for direct viewing spot display of letters and numbers, as shown in simplified perspective form in FIG. 3. A longitudinal heater strip 38 comprising a series of thermionic emitters generates electrons which are formed into a series of electron beams 40 modulated by a succession of grids 42. The beams enter a space between two facing plates, one a backplate 44 having a series of horizontal strip electrodes 46 thereon, and the opposite plate a glass faceplate 48 having a conductive layer 50 and a cathodoluminescent material deposited thereon. The potentials on the strip electrodes 46 and the conductive layer 50 are made equal, resulting in "practically an equipotential space" (quoted from column 3, lines 23-24 of the subject patent). The beams travel through the space 52 to a collector electrode 54. Reducing the voltage on a conductive strip causes the potential to become unequal and results in diversion of the beams toward the faceplate at the level of the strip, according to the disclosure. The device as shown would seem to lend itself to only the simplest of displays. Again, such a display would necessarily be small as the structure is not self-supporting.
In summing up, it appears that attempts to apply electron-beam picture tube technology to a flat panel display have been largely frustrated by one or both of such factors as the screen-size limitation dictated by the difficulty of providing internal envelope support in regions of beam excursion, and the need to utilize a high-energy beam to get adequate phosphor excitation. This need in turn dictates that beam control and modulating voltages be correspondingly high and out of the practical realm of utilization of transistor and integrated circuit technology.
______________________________________ Other Prior Art ______________________________________ 2,795,731 Aiken 3,177,395 Namordi et al 2,967,965 Schwartz 3,181,027 Geer 2,858,464 Roberts 3,379,912 Shanafelt 2,879,446 Aiken 3,435,269 Shanafelt 2,945,982 Foster 3,461,333 Havn 2,978,601 Aiken 3,395,312 Freestone et al 3,005,127 Aiken 3,683,224 Lea 3,904,923 Schwartz ______________________________________